The present invention relates to lithium oxyhalide cells and, more particularly, to voltage delay inhibitors which inhibit the growth of LiCl passive film on an electrode surface.
Two active prismatic Li/SOCl.sub.2 batteries are typically used in the development of Artillery Fired Atomic Projectiles (AFAP). One battery powers the telemetry system (TM battery) and the other powers a Projectile Event Timer circuit (PET battery). Both batteries must operate over a temperature range of -35.degree. C. to +55.degree. C. throughout the dynamic environment of the artillery gun barrel.
In artillery applications, a typical discharge lasts less than 180 seconds, and requires less than 10% of the energy capacity of each cell. The primary mode of electrochemical failure is voltage delay caused by the open-circuit growth of a passive film on the lithium anode during ambient storage, transportation and system assembly. In active Li/SOCl.sub.2 batteries, the Li anode reacts directly with the SOCl.sub.2 (thionyl chloride) electrolyte to form a LiCl passive film on the anode surface. This film is necessary to separate the anode and cathode of the cell. However, the film continues to grow thicker during the shelf-life storage of the battery. If the film becomes too thick, it may inhibit the kinetic performance of the anode once the battery is ultimately discharged. This results in a voltage delay and/or sustained low-voltage discharge.
Various approaches to eliminating this mode of failure include the use of: 1) electrolyte additives to minimize anode passivation, 2) a high-rate predischarge (at the time of manufacture) to initiate reproducible passive film growth, and 3) a specified controlled low-temperature shelf-life storage environment. Several substances have been added to Li/SOCl.sub.2 cells to mitigate voltage delay, including: LiAl; cyanoacrylate; epoxy (special formulation); SiF.sub.4 ; polyvinyl chloride; Li.sub.2 B.sub.10 Cl.sub.10, Li.sub.2 B.sub.12 Cl.sub.12 ; NbCl.sub.5 +Li.sub.2 S/Li.sub.2 O; LiGaCl.sub.4 ; So.sub.2 (with cobalt tetracarboxylated phenyl porphyrin); SiO.sub.2, TeO.sub.2 (with LiNbCl.sub.6); LiSO.sub.3 X (X being F.sup.-, Cl.sup.-, or Br.sup.-), SO.sub.3 ; and LiAl(SO.sub.3 Cl).sub.4 (TCSAL).
The precise mechanisms by which these additives inhibit LiCl film growth is not known. The kinetics of LiCl film growth in Li/SOCl.sub.2 cells can be monitored in situ by measuring the frequency dispersion of the complex impedance of the active cell. Appropriate mathematical analysis of the high frequency region of the impedance spectrum yields the ionic resistance and dielectric capacitance of the LiCl passive film. These parameters are related to film thickness. However, both parameters (resistance and capacitance) are also affected by structural defects such as grain boundaries and porosity of the passive film, the composition and concentration of the electrolyte, and the temperature of the cell. The resistance of the passive film controls the voltage delay in the cell discharge, and cells with low passive film resistances exhibit minor delays and high discharge voltage. Cells with high passive film resistance exhibit longer delays and lower voltage discharges. Passive film resistance is non-ohmic.
U.S. Pat. No. 4,547,441 discloses an electrochemical cell comprising a negative active material, and an electrolyte comprising a solute, a liquid oxyhalide solvent, and a mineral substance which reduces a voltage rise delay in the cell. The negative active material may be lithium. The mineral substance may be added to the electrolyte or may be reacted with a substance from the group comprising HSO.sub.3 X, SO.sub.3, H.sub.2 SO.sub.4 and mixtures thereof, wherein X may be chlorine, fluorine, bromine or iodine. The mineral substances may be LiAl(SO.sub.3 Cl).sub.4. LiAl(SO.sub.3 Cl).sub.4 (tetra-chlorosulfato-aluminate, lithium salt, or TCSAL) is disclosed as a voltage delay inhibitor in Li/SOCl.sub.2 cells. TCSAL is formed by the reaction of chlorosulfonic acid with SOCl.sub.2 and LiAlCl.sub.4. LiAlCl.sub.4 is a supporting electrolyte which is used in Li/SOCl.sub.2 cells. It is believed that the following reaction takes place: ##STR1## The HCl is removed by reflux prior to filling cells.
U.S. Pat. No. 4,309,490 discloses an electrochemical cell comprising a lithium anode, thionyl chloride, lithium tetrachloroaluminate electrolyte and sulfur dioxide to prevent growth of a lithium chloride crystal film on the lithium anode. A sufficient quantity of sulfur dioxide is employed in the cell to regulate the growth of the crystals on the lithium anode. The SO.sub.2 complexes with LiAlCl.sub.4 to form the complex LiAlCl.sub.4 *xSO.sub.2. Typically, the 1:1 complex (x=1) is used in Li/SOCl.sub.2 cells (e.g. LiAlCl.sub.4 *SO.sub.2).
U.S. Pat. No. 4,649,209 discloses a process of preparing methyl chlorosulfates, comprising the steps of: 1) reacting methylene chloride and sulfur trioxide; 2) adding a stabilizing agent such as sodium carbonate or sodium bicarbonate to allow distillation without decomposition of the reaction mixture; 3) distilling the stabilized reaction mixture by a two-stage fractional distillation procedure; and 4) recovering the methyl chlorosulfates. In the first stage, chloromethyl chlorosulfate is the primary product and in the second stage methylene bis(chlorosulfate) is the primary product. Methylene bis(chlorosulfate) is disclosed as an electrolyte for lithium batteries.